Flow instabilities in a liquid nitrogen evaporator, 2

Flow instabilities in a liquid nitrogen evaporator consisting of parallel vertical channels are studied experimentally and analytically both with nucleate boiling in the inlet section, the flow pattern of which transfers from bubbly flow to annular flow and with film boiling, from inverted annular flow to dispersed flow. As the channels are heated by electrical current, i.e., the wall heat flux is constant, both flow pattern transitions are obtained depending on the initial wall temperature for the same experimental conditions of the heat flux, the mass flux and the exit pressure. Density wave oscillations out of phase in the parallel channels are observed in both flow patterns and compared. Experimental results indicate that the flow with film boiling at the inlet section is stable for higher exit quality and its periods of oscillation are shorter than those with nucleate boiling at the inlet section. Linear stability analysis for the density wave oscillation predicts well the experimental stability criteria with nucleate boiling at the inlet section by using the previous drift flux models and those with film boiling by using a drift flux model formulated by analytical results obtained with a turbulent boundary layer theory for the inverted annular flow. (author)